... > control chart Options > Confidence Limits Charts T2, Generalized Variance
Specifies the confidence limit positions.
Dialog box items
Display confidence limits at
These percents: Enter one or more values or enter a column of values. Use proportion values (between 0 and 1) or percent values (between 1 and 100).
Place bounds on confidence limits
Lower confidence limit bound: Check to set the lower confidence limit bound, then type the lower limit (must be greater than 0) for the T2 chart. If the calculated lower confidence limit is less than the lower bound, Minitab draws a horizontal line labeled LB at the lower bound instead.
Upper confidence limit bound: Check to set the upper confidence limit bound, then type the upper limit (must be greater than 0) for the T2 chart. If the calculated upper confidence limit is greater than the upper bound, Minitab draws a horizontal line labeled UB at the upper bound instead.
To specify confidence limits
1 In control chart dialog box, click control chart Options. Click the Confidence Limits tab.
2 Do any of the following:
• Under Display confidence limits at, enter one or more values or enter a column of values in These percents.
Enter proportion values (between 0 and 1) or percent values (between 1 and 100).
• Under Place bounds on confidence limits, check Lower confidence limit bound, then type the lower confidence limit (greater than 0).
• Under Place bounds on confidence limits, check Upper confidence limit bound, then type the upper confidence limit (greater than 0).
3 Click OK.
Options − Stages
... > control chart Options > Stages Charts All charts except Z-MR
You can display stages in your process by drawing a "historical chart" − a control chart in which the control limits and center line are estimated independently for different groups in your data. Historical charts are particularly useful for comparing data before and after a process improvement.
Note With the following charts, you must have at least one subgroup with two or more observations: R, S, X-bar and R, and X-bar and S.
Dialog box items
Define stages (historical groups) with this variable: Enter the column that contains the stage indicators.
When to start a new stage
With each new value: Choose to start a new stage each time the value in the column changes.
With the first occurrence of these values: Choose to start a new stage at the first occurrence of a certain value, then enter the values. Enclose date/time or text entries in double quotes. You can enter the same value more than once;
Minitab treats each repeated value as a separate occurrence.
To display a historical chart
To define stages in your process, you must set up a column of grouping indicators. The indicators can be numbers, dates, or text. When executing the command, you can tell Minitab to start a new stage in one of two ways:
• Each time the value in the column changes
• At the first occurrence of one or more values
The column must be the same length as the data column (or columns, when subgroups are across rows).
1 In the control chart dialog box, click control chart Options.
2 Click the Stages tab.
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In Define stages (historical groups) with this variable, enter the column which contains the stage indicators.
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Under When to start a new value, do one of the following:
• To start a new stage each time the value in the column changes, choose With each new value.
• To start a new stage at the first occurrence of a certain value, choose With the first occurrence of these values.
Enter the values. Enclose date/time or text entries in double quotes. You can enter the same value more than once; Minitab treats each repeated value as a separate occurrence.
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If you like, use any dialog box options, then click OK.
Options − Box-Cox
... > control chart Options > Box-Cox Charts All except the Attributes charts
You can use the Box-Cox power transformation when your data are very skewed or when the within-subgroup variation is unstable to make the data "more normal." The transformation takes the original data to the power λ, unless λ = 0, in which case the natural log is taken. (λ is lambda.)
To use this option, the data must be positive. When you include or exclude rows using control chart Options > Estimate, Minitab only uses the non-omitted data to find lambda.
The Options subdialog box lists the common transformations natural log (λ = 0) and square root (λ = 0.5). You can also choose any value between − 5 and 5 for λ. In most cases, you should not choose a λ outside the range of −2 and 2. You may want to first run the command Stat > Control Charts > Box-Cox Transformation to help you find the optimal transformation value.
When you use this transformation, Minitab does not accept any values you enter in the Parameters tab for historical means or historical standard deviations.
Caution If you use Stat > Control Charts > Box-Cox Transformation to find the optimal λ value and choose to store the transformed data with that command, do not select the Box-Cox option when you make a control char;
doing so will double-transform the data.
Dialog box items
Box-Cox power transformation (W = Y**Lambda): Choose to use when your data are very skewed or when the within-subgroup variation is unstable.
Lambda = 0 (natural log): Choose to use the natural log of the data.
Lambda=0.5 (square root): Choose to use the square root of the data.
Optimal lambda: Choose to have Minitab search for an optimal value.
Optimal lambda for each stage (in a chart with stages): Choose to have Minitab search for optimal values for each stage.
Use overall standard deviation: Check to use the overall standard deviation in the estimation of the optimal lambda.
Other (enter value(s) between -5 and 5): Choose to transform the data using another lambda value or values, then enter the lambda values.
To do the Box-Cox power transformation
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In the control chart dialog box, click control chart Options. Click the Box-Cox tab.2
Check Box-Cox power transformation (W = Y**Lambda), then do one of the following:• Choose Lambda = 0 (natural log) to use the natural log of the data.
• Choose Lambda=0.5 (square root) to use the square root of the data.
• Choose Optimal lambda to have Minitab search for an optimal value for all stages.
• Choose Optimal lambda for each stage (in a chart with stages) to have Minitab search for optimal values for each stage.
• Choose Other (enter value(s) between -5 and 5) to transform the data using another lambda value or values, then enter one or more values.
For help choosing a lambda value, see the independent Box-Cox transformation command Stat > Control Charts >
Box-Cox Transformation.
3
Click OK.Options − Display
... > control chart Options > Display Charts All charts except Z-MR
Use to display the control chart by stage or by number of plotted points and to display the test results in the Session window.
Subgroups to display
Display all subgroups: Choose to display all subgroups.
Display last __ subgroups: Choose to display a specific number of subgroups, and enter the number of subgroups.
Split chart into series of segments for display purposes Do not split: Choose not to split the chart.
Each segment contains __ subgroups: Choose to split the control chart into subgroups of a specified number of points, and enter the number of subgroups for each segment.
Each segment corresponds to a stage (if chart has stages): Choose to split the control chart by stage or stage when you have specified stages in control chart Options > Stage.
Test results
Display test results in Session window: Check to display the test results in the Session window.
To display subgroups
1 In the control chart dialog box, click control chart Options. Click the Display tab.
2 Do one of the following:
• To display all subgroups in the control chart, under Subgroups to display, choose Display all subgroups.
• To display a specific number of the last subgroups, under Subgroups to display, choose Display last __
subgroups and enter the number of subgroups to display.
3 Click OK.
To split a control chart
1 In the control chart dialog box, click control chart Options. Click the Display tab.
2 Do one of the following:
• To split the chart into a specific number of subgroups, choose Each segment contains __ subgroups, and enter the number of subgroups for each segment.
• To split the chart by stage, choose Each segment corresponds to a stage (if chart has stages).
3 Click OK.
Options − Storage
Stat > Control Charts > control chart type > control chart > control chart Options > Storage Charts T2, T2 and Generalized Variance
Use to store your choice of statistics in the worksheet.
Dialog box items
Store these estimates for each chart
Means: Check to store the estimates of the means, one row for each historical stage.
Covariance matrices: Check to store the estimated covariance matrices, one row for each historical stage.
Store these values for each point
Point plotted: Check to store the plotted points, one row for each plotted point.
Center line value: Check to store the center line value, one row for each plotted point.
Control limit values: Check to store the control limit values, one row for each plotted point. Minitab stores one column for the lower control limit and one column for the upper control limit.
Stage: Check to store the stage, one row for each plotted point.
Subgroup size: Check to store the subgroup sizes, one row for each plotted point.
Test results: Check to store the results of any test performed, one row for each plotted point. Minitab assigns each plotted point one of three values: 1 (beyond upper control limit), -1 (beyond lower control limit), 0 (in control).
Decomposed Tsquared values: Check to store the decomposed T2 values.
To store statistics in the worksheet
1 Choose Stat > Control Charts > control chart type > control chart > control chart Options > Storage.
2 Check the statistics to store in the worksheet, then click OK.